In geophysics, knowledge of the structure of bedrock and rocks is advantageous in many applications whether with a view to drawing up maps of the bedrock, with the aim of searching for and tracking natural resources for example, to monitoring gravitational instabilities or indeed to monitoring sites the mechanical integrity of which is subject to potential random factors.
The characterization of the properties of bedrock employs indirect measuring techniques such as, for example, seismic imaging, electrical prospection or even gravimetry. Depending on the approach and measuring apparatus used, the obtained data allow, after study and processing, an image of the bedrock to be obtained.
Transmission tomography is a known imaging technique that allows the interior volume of an object (e.g. geological structure, work of art, industrial infrastructure) to be reconstructed on the basis of remote measurements taken from outside of the object.
In patent applications EP 0081314 and US 2008/0128604 A1, a plurality of detectors placed inside a bore well use tomographic analysis to determine local densities, this analysis exploiting electrical signals representative of the path of muons penetrating into the ground and intercepted by the detectors.
Muons are charged particles produced by the interaction of high-energy cosmic protons with the atmosphere. Muons, because of their high mass, which is about 207 times higher than that of electrons, their high speed (0.9997 c) and their insensitivity to the strong interaction, have a high penetrating power with respect to matter. Typically, they will propagate several hundred meters through rock, the depth reached essentially depending on their initial energy and on the density of the medium passed through. Thus, analysis of the number of muons received as a function of path makes it possible to gain information on bedrock density.
Devices such as those of the aforementioned patent applications require a plurality of detectors having a suitable geometry to be installed in order to collect the electrical signals produced by the detectors and to analyze them. Their use however remains limited to zones that have an accessibility that generates few constraints on bulk.
Moreover, it is also necessary to take into account the fact that the number of muons decreases with depth and that at certain depths the low muon flux may require long measurement times. For fluidic detectors, such as that of aforementioned patent application US 2008/0128604 A1, the measurement time is such that variations in the fluid injected into the detector may affect the stability of the measurement. Thus, the quality of the fluid used in a gaseous detector is critical to the achievement of a good system performance.
There is no known solution that allows bedrock density to be characterized from confined environments generating severe bulk constraints while allowing a measurement stability to be achieved. The present invention meets this need.